The present invention relates primarily to single-chip data processing devices, but also to microprocessors and to digital circuits generally, and to interface circuits.
In the present day, many products incorporate microprocessor based data processing circuits, for example to process signals, to control internal operation and/or to provide communications with users and external devices. To provide compact and economical solutions, particularly in mass-market portable products, it is known to include microprocessor functionality together with program and data storage and other specialised circuitry, in a custom "chip" also known as an application-specific integrated circuit (ASIC). Field Programmable Gate Arrays (FPGA) such as those made by Xilinx.TM., Actel.TM. and Altera.TM. may also be used to implement such solutions.
However, for various reasons, the integrated microprocessor functionality conventionally available to an ASIC designer tends to be the same as that which would be provided by a microprocessor designed for use as a separate chip. The present inventors have recognised that this results in inefficient use of space and power in the ASIC solution, and in fact renders many potential applications of ASIC technology impractical and/or uneconomic.